1. Field of the Invention
The present invention relates to a piezoelectric component and a method of manufacturing the piezoelectric component. In the piezoelectric component, a periphery of a piezoelectric element is covered by an elastic material, and the circumference of the elastic material and the piezoelectric element is covered by an outer-cladding resin.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 1-228310 describes a piezoelectric component in which a periphery of a piezoelectric element having end electrodes connected to lead terminals is covered by an elastic material such as a silicone rubber, and the circumference thereof is sealed by an outer-cladding resin.
In order to manufacture such a piezoelectric component, first, a lead terminal is attached to a piezoelectric element. Then, a silicone rubber layer is provided on the piezoelectric element by the dipping the piezoelectric element in a liquid-state silicone rubber and hardening the rubber layer by applying heat. Further, an epoxy resin layer is provided on the exterior of the silicone rubber via a dipping method. Then, the epoxy resin is hardened by heat to obtain an outer-cladding resin.
The epoxy resin used for forming the outer-cladding resin layer contracts when it is hardened by heat, and thereby a stress caused by the outer-cladding resin which contracts and compresses the interior of the piezoelectric component is generated. If the amount of stress which contracts and compresses the piezoelectric element in the piezoelectric component changes, the oscillation characteristics thereof change. The silicone rubber is used because it is able to prevent the change of the oscillation characteristics of the piezoelectric element, and has a damping effect which suppresses the waveform distortion of the piezoelectric element.
To reduce the stress generated in the piezoelectric element by the outer-cladding resin, preferably, the difference between the edge portion of the piezoelectric element and the outer-cladding resin i.e., the minimum thickness of the silicone rubber, is larger than the contraction amount of the outer-cladding resin when it is hardened, and further, is larger than the difference between the thermal expansions of the outer-cladding resin and of the piezoelectric element.
However, the dipping method for forming the liquid-state silicone rubber was not satisfactory for obtaining a required thickness of the silicone rubber at the end portions of the piezoelectric element, because of the surface tension of the silicone rubber. Therefore, the temperature characteristics of the piezoelectric components are largely varied by the stress generated in the piezoelectric element by the outer-cladding resin.
To overcome the above described problems, preferred embodiments of the present invention provide a piezoelectric component and a method of manufacturing the piezoelectric component, which greatly reduces the stress generated in the piezoelectric element by the outer-cladding resin while the function of damping the piezoelectric element is maintained.
One preferred embodiment of the present invention provides a piezoelectric component, including a substantially rectangular shaped piezoelectric element, a first elastic material covering at least a pair of end portions of the piezoelectric element, the pair of end portions including edge parts of the piezoelectric element, a second elastic material covering the entire piezoelectric element and the first elastic material, and an outer-cladding resin covering the whole circumference of the piezoelectric element covered with the second elastic material.
In the above described piezoelectric component, the end portions of the piezoelectric element, where it is most difficult to obtain a sufficient thickness of an elastic material, is covered by the first elastic material. Further, the periphery of the first elastic material is covered by the second elastic material. That is, two elastic materials are provided at the edge parts of the piezoelectric element, where the piezoelectric element is most easily affected by the contraction stress imparted by the outer-cladding resin. Therefore, even when the contraction stress imparted by the outer-cladding resin varies according to temperature changes, a change in oscillation characteristics caused by the change of temperature is prevented because the first and second elastic materials having sufficient thickness absorb any changes of stress. Further, the damping effect is maintained by the second elastic material to thereby prevent a waveform distortion or other harmful effects from being generated.
The modulus of elasticity and the hardness of the first and second elastic materials are determined in accordance with respective required characteristics. In the case of the second elastic material. For example, it is preferable to set the hardness equal to or less than 28 in xe2x80x9cShore Hardness A.xe2x80x9d
Another preferred embodiment of the present invention provides a method of manufacturing a piezoelectric component, including the steps of forming an unhardened first elastic material partially on at least a pair of end portions of a piezoelectric element, the pair of end portions including edge parts of the piezoelectric element, then hardening the first elastic material, forming an unhardened second elastic material on the entire circumference of the piezoelectric element and the first elastic material, then hardening the second elastic material and forming an unhardened outer-cladding resin on the entire circumference of the second elastic material covering the piezoelectric element and the first elastic material, then hardening the outer-cladding resin.
By the above described method, the piezoelectric component according to preferred embodiments of the present invention can be manufactured easily.
For forming the unhardened first and second elastic materials, not only a dipping method but also using a soldering iron, dispenser, or other suitable method can be utilized. Similarly, a dipping method or other methods can be utilized for forming the unhardened outer-cladding resin.
Although the first elastic material and the second elastic material may be the same, it is preferable to make the thixotropic index (call it a thixo index hereafter) of the first elastic material larger than the thixo index of the second elastic material. The thixotropic property is one of the characteristics of fluid, and is defined as a property in which a viscosity varies nonlinearly depending on a shearing stress. It is easy to achieve sufficient thickness of the elastic material when forming the elastic material on the piezoelectric element if the thixo index of the elastic material is large. Therefore, if the thixo index of the first unhardened elastic material is made larger than the thixo index of the second unhardened elastic material, sufficient film thickness at the both edge parts of the piezoelectric element can be obtained. On the other hand, there is no problem even when the thixo index of the second elastic material is small, because it is easy to coat the center portion of the piezoelectric element using the second elastic material, and this construction is hardly affected by heat.
Preferably, the thixo index of the first and second elastic material is preferably larger than about 1.7. If the thixo index is equal to or less than about 1.7, it is hard to achieve a sufficient thickness of the elastic material at the end portions of the piezoelectric element because of a surface tension of the elastic material.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.